System and method for multiple layer satellite communication

ABSTRACT

A system and method are disclosed that may include a system and method for transmitting user data from an initial satellite in a first constellation located conveniently to an origination user terminal to a destination satellite in the first constellation located conveniently to a destination user terminal, the method including transmitting the user data over an optical link from the initial satellite in the first constellation to a first satellite within a second satellite constellation; conveying the user data over an optical link from the first satellite in the second constellation to a second satellite in the second constellation; and receiving the user data from the second satellite in the second constellation at the destination satellite in the first constellation.

BACKGROUND OF THE INVENTION

This application relates in general to communications and in particularto satellite communications.

Satellite communication systems provide various benefits to consumers ofcommunication services such as telephony, internet communications, andtelevision communications among others. Various satellite systems arecurrently available, which are discussed below.

Currently, satellite communication is implemented using satellitesarranged in a single constellation though distributed over differentplanes, all having the same orbit geometry. Earth stations in the formof user terminals communicate with one or more satellites using RadioFrequency (RF) communication links. Thereafter, the satellite systemcommunicates with another earth-based terminal which is linked in to abroader network, such as the Internet, a Cable television system, orother communication network. In this manner, an otherwise isolated userterminal can be placed in communication with a global network using asatellite system as a communication intermediary.

Current satellite communication systems suffer from the drawback thatcommunication bandwidth is limited by the bandwidth of each satellite inthe system. Additional satellites may be deployed to address thisconcern. However, this solution adds considerable expense to the overallsystem due to the cost of constructing, launching, and maintaining theoperation of ever greater numbers of satellites. Limits on theavailability of communication spectrum may impose a limit on the numberdifferent wavelengths that the system can communicate over at one time.Moreover, in satellite systems with large numbers of satellites that useinter-satellite communications, it is difficult for two satellites tocommunicate with one another where one satellite is moving northboundalong one line of longitude and the other satellite is moving southalong a neighboring line of longitude because the closing speed (therate at which the satellites are approaching one another) is at a levelthat makes it difficult for satellites to accurately track one anotherat the level required for the pointing of communications links.

Accordingly, there is a need in the art for a satellite communicationsystem that can accommodate greater communication bandwidth, and thatwill allow better inter-satellite communications, without imposing anundue cost burden and which can operate within the constraints imposedby the limited communication spectrum available to modern satellitesystems.

SUMMARY OF THE INVENTION

According to one aspect, the invention is directed to a communicationsystem that may include a first constellation of satellites traveling ina substantially polar orbit and operable to communicate with earth-baseduser terminals over respective radio frequency (RF) links; and a secondconstellation of satellites traveling in an inclined orbit operable tocommunicate with selected satellites in said first constellation overrespective optical communication links.

Other aspects, features, advantages, etc. will become apparent to oneskilled in the art when the description of the preferred embodiments ofthe invention herein is taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purposes of illustrating the various aspects of the invention,there are shown in the drawings forms that are presently preferred, itbeing understood, however, that the invention is not limited to theprecise arrangements and instrumentalities shown.

FIG. 1 is a schematic view of a multiple layer satellite communicationsystem in accordance with an embodiment of the present invention;

FIG. 2 is a perspective view of a plurality of satellite constellationsorbiting the earth, in accordance with an embodiment of the presentinvention;

FIG. 3 is a block diagram of a data center in communication with one ormore satellite constellations in accordance with an embodiment of thepresent invention;

FIG. 4A is a schematic representation of a portion of a satellite orbitin proximity to the equator, in accordance with an embodiment of thepresent invention;

FIG. 4B is a schematic representation of a portion of a satellite orbitin proximity to the equator, in accordance with an embodiment of thepresent invention; and

FIG. 5 is a block diagram of a computer system useable in conjunctionwith an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, for purposes of explanation, specificnumbers, materials and configurations are set forth in order to providea thorough understanding of the invention. It will be apparent, however,to one having ordinary skill in the art that the invention may bepracticed without these specific details. In some instances, well-knownfeatures may be omitted or simplified so as not to obscure the presentinvention. Furthermore, reference in the specification to phrases suchas “one embodiment” or “an embodiment” means that a particular feature,structure or characteristic described in connection with the embodimentis included in at least one embodiment of the invention. The appearancesof phrases such as “in one embodiment” or “in an embodiment” in variousplaces in the specification do not necessarily all refer to the sameembodiment.

FIG. 1 is a schematic view of a multiple layer satellite communicationsystem 50 in accordance with an embodiment of the present invention.FIG. 1 is provided to provide a conceptual overview of the operation ofan embodiment of the present invention. In the embodiment shown, userterminals 102-a and 102-b (collectively user terminals 102) communicatedirectly with satellites 202-a and 202-b, respectively, in constellation1 200 (the RF constellation) and indirectly using constellation 2 300(the optical constellation), thereby providing a dual-layer satellitesystem.

An embodiment of the present invention employs a multiple-constellationsatellite system that uses RF transmission links to transmit user datathrough the ambient data-transmission obstructions prevalent at lowaltitudes, and which uses optical communication through free space tohandle high-bandwidth data transmission between satellites in the RFconstellation 200 in locations suitable for communication with theorigination and destination user terminals 102, respectively. In thismanner, an embodiment of the present invention can use RF communicationfor the limited purpose of piercing through data-path obstructions dueto ambient conditions close to the surface of the earth (i.e. near-earthcommunication), but which then uses optical communication to conducthigh-speed communication through free space once the user data is freeand clear of the afore-mentioned data-path obstructions. The opticalconstellation 300 may thus be used as a high-speed communications trunkline to transmit data to a destination RF satellite 102 convenient to adestination user terminal 102, thereby alleviating data trafficcongestion in the RF constellation 200.

An embodiment of the present invention enables user terminals 102 on theearth 100 to communicate directly, over RF links, with an RFconstellation 200, thereby enabling the user terminal to transmit datathrough rain, clouds, and other weather-related obstacles due to therobustness RF communication signals. Thereafter, the RF constellationmay further transmit the user data to the optical constellation 300using an optical link. The optical constellation 300 can then convey theuser data to a satellite 302 in the optical constellation 300 mostconvenient to the destination user terminal to which the user data isbeing transmitted. The data may then be transmitted back to the RFconstellation 200 and in turn to the destination user terminal.

The above arrangement preferably operates to alleviate data trafficcongestion within satellites 202 in the RF constellation 200 bytransferring the bulk of the data traffic to the optical constellation300. Moreover, the use of the multiple-layer satellite system (which mayhave two, three or more layers) may be operated in a manner that isopaque to users and user terminals 102 on the earth 100, therebyavoiding any need to alter communication protocols or addressing data toaccommodate the operating characteristics of the multiple-layersatellite system.

The number of satellites deployed in each of the first constellation 200and the second constellation 300 may be variable and scalable based onthe needs of the system 50. However, in a preferred embodiment, oneoperating principle relating to the number of satellites 302 to bedeployed within constellation 300 is that the number of satellites 302should be sufficient to enable any satellite 202, in constellation 200,to communicate with any other satellite 202, using the satellites 302 inconstellation 300 as communication intermediaries. This is essentiallyequivalent to saying that there should be fullcommunication-connectivity among the satellites 202 of constellation200. Once full communication-connectivity among the satellites 202 ofconstellation 200 is established, it follows that each user terminal 102in communication with any satellite 202 will be able to communicate withany other user terminal 102, within system 50, using constellation 200and possibly constellation 300 as communication intermediaries.

We turn now to a detailed discussion of the specific embodiment shown inFIG. 1. System 50 may include user terminals 102-a and 102-b on theearth 100; constellation 1 (the RF constellation) moving in a polarorbit at an altitude of about 950 kilometers (km) and includingsatellites 202-a and 202-b (collectively RF satellites 200) as partthereof; constellation 2 (the optical constellation) 300 moving in aninclined orbit at an inclination angle of about 57.5 degrees at analtitude of about 1350 km and including satellites 302-a and 302-b(collectively optical satellites 300). It is noted that the term “RFsatellites” is merely a useful abbreviated term for referring tosatellites 200. In the embodiment shown in FIG. 1 (among otherembodiments), RF satellites 200 include both RF and opticalcommunication ports, which will be discussed in greater detail elsewherein this document.

It is noted that the present invention is not limited to having theconstellations employ the altitudes and inclination angles mentionedherein. For instance, in alternative embodiments, the RF constellation200 could operate any altitude between 750 km and 1000 km, and with aninclination angle between 85 degrees and 95 degrees. Moreover, inalternative embodiments, the optical constellation 300 could operate ataltitudes between 1150 km and 1550 km. Moreover, the invention is notlimited to using only two constellations having different orbitgeometries. Otherwise stated, the invention is not limited to asatellite system having only two “layers.” Specifically, three or moresatellite layers could be employed. More broadly, the concept disclosedherein of using a one or more satellite constellations to conductcommunication with earth-based user terminals using RF datacommunication links and a one or more other satellite constellations toserve as high-speed communication trunk lines may be employed with anydesired combination of orbit altitudes and orbit angle inclinations (asmeasured with respect to the equator). Moreover, although in oneembodiment satellites 302 may use optical communication alone, theinvention is not limited to employing only optical communication inconstellation 300, but may instead use a combination of RF and opticalcommunication, or RF communication alone, and all such variations areintended to be included within the scope of the present invention.

Having discussed the communication devices, we now turn to thecommunication links. In one embodiment, communication between userterminals 102 and RF satellites 202 may occur over respective RF links120 and may occur at a range of rates. On each RF data link 120, thedata rate may be anywhere from 0 to 10 gigabits per second (Gbps).Communication between the RF satellites 202 and the optical satellites300 may occur over optical communication links 220 and preferably at arate of 10 Gbps. Communication between the optical satellites 300 mayoccur over respective optical communication links 320, preferably at arate of 40 Gbps. It is noted that, in this embodiment, data link 320carries data within constellation 300 at about four times the rate ofthe links from the earth 100 to the RF constellation 200 and the linksfrom the RF constellation 200 to the optical constellation 300. This isbecause data traffic from multiple RF satellites 200 (and what may bemany earth-based user terminals 102 for each such RF satellite) may bemultiplexed onto the data traffic paths within constellation 300.

While various specific data communication rates were specified for theembodiment shown in FIG. 1, the present invention is not limited to thelisted data rates. The RF communication may take place at a range ofrates above or below 10 Gbps. Moreover, the communication between the RFsatellites 202 and the optical satellites 302 may occur at rates aboveor below 10 Gbps. Communication occurring over data links 320, betweenoptical satellites 302 within constellation 300, may occur at ratesabove or below 40 Gbps.

We now consider an example of a data communication scenario using theembodiment shown in FIG. 1. In this example, a user at user terminal102-a on the earth 100 wishes to send data to user terminal 102-b, whichfor the sake of this example is presumed to very distant from userterminal 102-a, and thus beyond the communication range of satellite202-a. User terminal 102-a may send its user data along link 120 tosatellite 202-a within the RF constellation 200. Thereafter, satellite202-a may further transmit the user data over optical communication link220 to satellite 302-a within optical constellation 300.

For the sake of this part of the discussion, it is presumed that system50 includes data processing equipment (an exemplary version of which isshown in FIG. 3) that is operable to identify (a) a satellite 202-bwithin constellation 200 that is most suitable for conducting the lastleg of the data communication path to user terminal 102-b; and in turn(b) to identify the optical satellite 302-b most suitable fortransmitting the user data along the second-to-last leg of thecommunication path, to satellite 202-b.

Upon receiving the user data, satellite 302-b preferably transmits thedata to satellite 202-b in the RF constellation. Thereafter, satellite202-b preferably transmits the user data to user terminal 102-b on theearth 100. In one embodiment, the shift of the data communicationpayload from the RF constellation 200 to the optical constellation 300may occur so as to be completely opaque to user terminal 102-b and ahuman user of user terminal 102-b.

FIG. 2 is a perspective view of a satellite communication system 50 inaccordance with an embodiment of the present invention.

FIG. 2 is a perspective view of a plurality of satellite constellations200, 300 orbiting the earth 100, in accordance with an embodiment of thepresent invention. The satellites of constellation 200, which are movingalong a polar orbit, are arranged along various different lines oflongitude over the earth 100, with the satellites along each such lineof longitude forming a sub-constellation of constellation 200. Due tospace limitations, only sub-constellations 200-a and 200-b are calledout with reference numerals. Satellites in constellation 300 may bedistributed over several inclined planes. Due to space limitations onlytwo such planes are called out with reference numerals in FIG. 3: 300-aand 300-b.

In one embodiment, the RF constellation 200 may include about 1250satellites along with a few (between 50 and 100) spare satellites. TheRF satellites 200 may communicate using respective RF links 120 withuser terminals 102 on the earth 100, and with the optical satellites 300over optical communication links 220.

The satellites 202 in the RF satellite constellation 200 may be about 40cm (15.7″) long, and about 13.6 cm (5.4″) deep, and may weigh about 75kilograms (kg). In one embodiment, the RF satellites 202 may have ahexagonal shape. However, the present invention is not limited to usingRF satellites 202 with the dimensions and shapes discussed above.

In one embodiment, each RF satellite 202 microwave forms nineteen (19)individual beams that form coverage regions on the surface of the earth100 having a cellular honeycomb pattern. The RF satellites 202 may bespaced to adjust the size and location of these honeycomb to achievefull earth 100 surface coverage.

With reference to FIG. 2, when traveling along a polar orbit, thesatellites 202 travel along lines of longitude that are farthest apartat the equator, with overlapping coverage areas as they approachlatitudes approaching the north or south poles. Overlapping coveragearising from the proximity of the coverage areas of the RF satellites202 preferably enables greater data transmission throughput to morenortherly, heavily populated areas such, but not limited to, the UnitedStates, Europe, and China.

The rate of data communication throughput is the product of the width ofthe frequency band available for data transmission and the number ofbits per hertz. With more than 2.0 Ghz (gigahertz) of forward spectrum,embodiments of the present invention can operate using relatively lowpower levels to achieve efficiencies such as 1 bit/hz (one bit perhertz) or less, and still yield economically viable throughput. The useof lower power levels enables the resulting satellite design to besmaller and less expensive. Moreover, when the satellites are smallerand less expensive, it becomes possible to launch more satellites witheach launching operation, and to do so at lower cost.

Each satellite has an Orbit Average Power (OAP) of approximately 180watts. This allows the satellite to operate during the longest eclipseswith the average throughput discussed above. The above may be achievedby installing a battery having energy storage suitable for providing 180watts of power for a period of time equaling or surpassing the durationof the longest solar eclipse the satellite can experience.

Satellite Communication Ports

In one embodiment, each RF satellite 202 can route packets from any ofits communication ports to any port on another satellite with which itis communicating. Satellite 202 preferably includes an internal switchthat supports nineteen (19) separate RF beams and two optical ports.Otherwise stated, each RF satellite 202 serves as an Ethernet-typeswitch, with nineteen ports operable to communicate over RF links withuser terminals 102 on the earth 100, and two optical ports operable tocommunicate with satellites 302 in the optical constellation 300.

FIG. 3 is a block diagram of a data center 400 in communication with thesatellite constellations 200, 300 and with the Internet 600 inaccordance with an embodiment of the present invention. In theembodiment of FIG. 3, system 50 may include satellite layers(constellations) 200 and 300, user terminal 102 (on the earth) which maybe in communication with data center 400 (on the earth), which may inturn be in communication with the Internet 600.

Data center 400 may include a variety of computing equipment includingone or more components of computer system 600, which is described ingreater detail elsewhere herein. Data center 400, and one or more otherdata centers having comparable facilities, may be used to acquire andaccumulate data concerning the operation of system 50. Moreover, datacenter may generate and transmit data to the various satellites 202 and302 in constellations (layers) 200 and 300, respectively, relating tothe control of energy consuming devices aboard each satellite, and/orthe devices (whether ground-based user terminals or other satellites)that each satellite will communicate with, and which port on thesatellite is to be used for such communication.

The control decisions relating to the selection of satellites and/oruser terminals that any communication device (i.e. a satellite or userterminal) should communicate with may be conducted by a data center 400,by the device itself, or by a combination of the two. Moreover, the dataacquisition and control functions of communication system 50 may bedistributed over a range of computing devices on the earth 100, withinsatellite constellations 200, 300, or within a combination of theforegoing.

When a user terminal 102 is coupled to the Internet 600, as isillustrated in FIG. 3, the RF satellite 202 may redirect the internettraffic from data center 400 to a predetermined destination according toa predetermined data path, which is sometimes referred to in thecommunications industry as “bent-pipe” transmission. This could includea second user terminal 102 that is in communication with RF satellite202, but that is not directly connected to Data Center 400.Alternatively, RF satellite 202 may transmit the data over an indirectpath, that may include transmitting the data to a satellite 302 withinoptical satellite constellation 300, then, if needed, through opticalconstellation 300 to a suitably located optical satellite 302,thereafter to a suitably located RF satellite 202, and then to adestination user terminal 102, which might be in communication with adata center 400.

In one embodiment, user terminals 102 will be installed at as many datacenters as possible to ensure the majority of data traffic stays on thenetwork of system 50, which would allow system 50 to better manage thecustomer experience.

Optical Layer Design

In one embodiment, the Optical Layer (constellation) 300 may be a meshnetwork system backplane. The optical satellites 302 may orbit in aBallard Rosette orbit at a 57.5 degree inclination angle. The opticalconstellation 300 may include about 200 satellites that may be dividedamong ten planes, all at substantially the same inclination angle. Theremay be 20 satellites in each of the 10 planes. However, other, unequaldistributions of the satellites among the various planes could beimplemented. Preferably, each optical satellite 302 may communicate withfour other optical satellites 302 and eight RF satellites 202.

FIG. 2 shows an example of US-Brazil and US-Africa optical links. Sincefree space optical communication is about 30% faster than communicationthrough fiber-optic cable, embodiments of the present invention may beable to provide the lowest latency long-haul data communication networkamong all commercially available options. For example, simulations haveshown that an embodiment of the present invention can reduce theNairobi-MTV RTT (Round-Trip delay Time) latency from its current 245 msto 140 ms.

The communication from one optical satellite 302 to another mayinitially be designed to operate at 10 Gbps per wavelength, which whenusing four wavelengths, provides a total data throughput of 40 Gbps.Communication over optical links between the optical satellites 302 andthe RF satellites 202 may initially operate at a rate of 10 Gbps using asingle, tunable wavelength. It is anticipated that the link speeds willincrease over time with technology improvements and iterative systemrefreshes.

Spectrum

In one embodiment, 2.05 Ghz of Ku band Space-to-Earth and 1.5 GhzEarth-to-Space, and 900/1075 Mhz of C band bandwidth may be employed.FIGS. 4A and 4B are schematic representations of portions of satellite200 orbits in proximity to the equator 110, in accordance with anembodiment of the present invention. One embodiment of the presentinvention may communicate within a portion of the GSO spectrum bymaintaining an angular separation from the GSO communication beam 152 toprevent interference. FIG. 4A shows GSO earth station 150 and its lineof focus 152. Starting at a beam orientation about six degrees away fromthe GSO beam 152, non-GSO communication (i.e. beam 120) can be usedwithout interfering with GSO communication. The amount of transmissionpower that can be used without interfering with GSO communicationbecomes progressively greater with increasing angular separation of thenon-GSO beam 120 from the GSO beam 152. Once the separation angle(between the GSO beam and the non-GSO beam) is sufficiently large, theneed to coordinate transmission schedules with GSO transmissionsessentially disappears.

With reference to FIG. 4A, with a beam 32 degrees away from the equator110, an embodiment of the system disclosed herein can maintain over 37degrees of angular separation/isolation of RF satellite 200 beam 120from the GSO communication beam 152, giving system 50 the ability tooperate at very high power levels without incurring any risk ofinterference with GSO beam 152.

As the RF satellites 200 of system 50 approach the equator 110, theseparation angle between the beams of satellites 200 and the GSO beam152 declines, and we allow the satellites 200, 300 to tilt forward,while maintaining a 19-degree angular separation between the GSO andnon-GSO beams, while still reaching the equator.

FIG. 5 is a block diagram of a computing system 500 adaptable for usewith one or more embodiments of the present invention. For instance, acomputing system incorporating one or more of the components (depictedwith individual blocks in FIG. 5) of system 500 may be incorporatedwithin one or more data centers 400. Moreover, computing systemsincorporating one or more of the components of system 500 may beincorporated within any of user terminals 102, satellites 202 of the RFconstellation 200, and/or satellites 302 of the optical constellation300.

In computing system 500, central processing unit (CPU) 502 may becoupled to bus 504. In addition, bus 504 may be coupled to random accessmemory (RAM) 506, read only memory (ROM) 508, input/output (I/O) adapter510, communications adapter 522, user interface adapter 506, and displayadapter 518.

In an embodiment, RAM 506 and/or ROM 508 may hold user data, systemdata, and/or programs. I/O adapter 510 may connect storage devices, suchas hard drive 512, a CD-ROM (not shown), or other mass storage device tocomputing system 500. Communications adapter 522 may couple computingsystem 500 to a local, wide-area, or global network 524. User interfaceadapter 516 may couple user input devices, such as keyboard 526, scanner528 and/or pointing device 514, to computing system 500. Moreover,display adapter 518 may be driven by CPU 502 to control the display ondisplay device 520. CPU 502 may be any general purpose CPU.

It is noted that the methods and apparatus described thus far and/ordescribed later in this document may be achieved utilizing any of theknown technologies, such as standard digital circuitry, analogcircuitry, any of the known processors that are operable to executesoftware and/or firmware programs, programmable digital devices orsystems, programmable array logic devices, or any combination of theabove. One or more embodiments of the invention may also be embodied ina software program for storage in a suitable storage medium andexecution by a processing unit.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

The invention claimed is:
 1. A communication system, comprising: a firstconstellation of satellites traveling in an inclined orbit and operableto communicate with earth-based user terminals over respective radiofrequency (RF) links; a processor operable to monitor separation anglesbetween beams of said first constellation of satellites and a GSO beamand maintain each of the separation angles by at least a selected numberof degrees by temporarily tilting at least some satellites in the firstconstellation of satellites to move the beams of the respective RF linksaway from the GSO beam to avoid an interference with the GSO beamwithout deviating from the inclined orbit; and a second constellation ofsatellites traveling in an inclined orbit at a higher altitude than thefirst constellation and operable to communicate with selected satellitesin said first constellation over respective optical communication links.2. The communication system of claim 1 wherein satellites in the firstconstellation orbit at an altitude between 600 and 950 kilometers (km).3. The communication system of claim 1 wherein satellites in the secondconstellation orbit at an altitude of between 1100 and 1400 km.
 4. Thecommunication system of claim 1 wherein satellites in the secondconstellation travel along an orbit inclined at an angle between 35degrees and 60 degrees.
 5. The communication system of claim 1 whereinsatellites in the first constellation travel along an orbit inclined atan angle between 60 degrees and 90 degrees.
 6. The communication systemof claim 1 wherein the first constellation includes between 200 and 1500satellites.
 7. The communication system of claim 1 wherein eachsatellite in the second constellation is able to communicate with atleast two satellites in the second constellation and at least twosatellites in the first constellation.
 8. The communication system ofclaim 1 wherein each satellite in the first constellation comprises:multiple RF ports for communication with earth-based user terminals. 9.The communication system of claim 1 wherein each satellite in the firstconstellation comprises: at least two optical communication ports forcommunicating with one or more satellites in the second constellation.10. The communication system of claim 1 wherein any satellite in thefirst constellation communicates with any other satellite in the firstconstellation using the satellites in the second constellation ascommunication intermediaries.
 11. A communications system comprising: afirst satellite constellation in orbit around the earth and operable tocommunicate with a plurality of user terminals on the earth over radiofrequency (RF) links; a processor operable to monitor separation anglesbetween beams of said first satellite constellation and a GSO beam andmaintain each of the separation angles by at least a selected number ofdegrees by temporarily tilting at least some satellites in the firstsatellite constellation beams of the respective RF links away from theGSO beam to avoid an interference with the GSO beam; and a secondsatellite constellation operable to provide a high-speed communicationtrunk line between a data-path origination satellite and a data-pathdestination satellite within said first satellite constellation.
 12. Thecommunications system claim 11 wherein a majority of a set ofcommunications links between the first constellation and the secondconstellation are optical.
 13. The communications system of claim 11wherein a majority of a set of communication links between thesatellites of the second constellation are primarily optical.
 14. Thecommunications system of claim 11 wherein a majority of thecommunication links between satellites in the second constellation anduser terminals on the earth use radio frequency (RF) communication. 15.The communications system of claim 11 wherein the second constellationorbits at a higher altitude than the first constellation.
 16. Thecommunication system of 11 where the satellites of the firstconstellation include the ability to route traffic between twoindividual user terminals without passing the data traffic to asatellite in the second constellation.
 17. A method for transmittinguser data from an initial satellite in a first constellation to adestination satellite in the first constellation, the method comprisingthe steps of: (a) transmitting the user data from thefirst-constellation initial satellite to a first satellite in a secondconstellation, the second constellation having a different orbitaltitude than that of the first constellation, the first and secondconstellations both including a plurality of satellites and both havingnon-geostationary orbits; (b) identifying a second satellite in thesecond constellation that is most suitable for communication with thefirst-constellation destination satellite; (c) transmitting the userdata from the first satellite in the second constellation to theidentified second satellite in the second constellation; (d)transmitting the user data from the second satellite in the secondconstellation to the first-constellation destination satellite; and (e)monitoring separation angles between beams of said first constellationof satellites and a GSO beam and maintaining each of the separationangles by at least a selected number of degrees by temporarily tiltingat least some satellites in the first constellation of satellites tomove the beams of the respective RF links away from the GSO beam toavoid an interference with the GSO beam.
 18. The method of claim 17further comprising the steps of: the first-constellation initialsatellite receiving the user data from a first user terminal over aradio frequency (RF) link; and transmitting the user data from thefirst-constellation destination satellite to a second user terminal. 19.The method of claim 17 wherein the step of transmitting the user datafrom the first-constellation initial satellite to the first satellite inthe second constellation is conducted over an optical communicationslink.
 20. The method of claim 17 wherein the step of: transmitting theuser data from the first satellite in the second constellation to theidentified second satellite in the second constellation is performedover an optical communications link.
 21. The method of claim 17 furthercomprising: directing satellites in the first constellation along anorbit having a first inclination angle; and directing satellites in thesecond constellation along an orbit having a second inclination anglediffering by least five degrees from the first inclination angle. 22.The method of claim 17 wherein said transmitting steps (a), (c), and (d)are operable to alleviate data traffic congestion in said firstconstellation.
 23. A method for transmitting user data from an initialsatellite in a first constellation located conveniently to anorigination user terminal to a destination satellite in the firstconstellation located conveniently to a destination user terminal, themethod comprising the steps of: transmitting the user data over anoptical link from the initial satellite in the first constellation to afirst satellite within a second satellite constellation; conveying theuser data over an optical link from the first satellite in the secondconstellation to a second satellite in the second constellation;receiving the user data from the second satellite in the secondconstellation at the destination satellite in the first constellation;and monitoring separation angles between beams of said firstconstellation of satellites and a GSO beam and maintaining each of theseparation angles by at least a selected number of degrees bytemporarily tilting at least some satellites in the first constellationof satellites to move the beams of the respective RF links away from theGSO beam to avoid an interference with the GSO beam.
 24. The method ofclaim 23 wherein all of the satellites are in non-geostationary orbits.25. The method of claim 23 further comprising: receiving the user datafrom the origination user terminal over a radio frequency (RF) link atan initial satellite within the first satellite constellation.
 26. Themethod of claim 23 further comprising: transmitting the user data fromthe destination satellite in the first constellation to the destinationuser terminal.
 27. The method of claim 23 further comprising:controlling communication at said initial and destination satellites insaid first constellation and said first and second satellites in saidsecond constellation using a data center on the earth.
 28. The method ofclaim 23 wherein the steps of transmitting, conveying, and receivingcorrespond to using the second constellation as a space-basedcommunications trunk line for said first constellation.
 29. The methodof claim 23 wherein the conveying step comprises: transmitting the userdata through a sequence of satellites within the second constellationlocated between the first and second satellites in the secondconstellation.
 30. The method of claim 23 wherein the conveying stepcomprises: transmitting the user data directly from the first satellitein the second constellation to the second satellite in the secondconstellation.